Electronic memory device having an electrode made of a soluble material

ABSTRACT

An electronic device includes a first electrode made of an inert material; a second electrode made of a soluble material; a solid electrolyte made of an ion-conductive material, wherein the first and second electrodes are in contact respectively with one of the faces of the electrolyte, either side of the electrolyte, wherein the second electrode supplies mobile ions flowing in the electrolyte towards the first electrode, to form a conductive filament when a voltage is applied between the first and second electrodes. The second electrode is a confinement electrode that includes an end surface in contact with the electrolyte which is less than the available surface of the electrolyte, such that confinement of the contact area of the confinement electrode on the solid electrolyte is obtained.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from French PatentApplication No. 1261260 filed on Nov. 27, 2012, the entire content ofwhich is incorporated herein by reference.

FIELD

The present invention relates to an electronic memory device; theinvention relates more particularly to the field of rewritable memories,and more specifically that of CBRAM or “Conductive Bridging RAM”memories.

BACKGROUND

Depending on the applications and the desired performancespecifications, different types of memories are used.

Memories of the SRAM, or static RAM, type, thus have ultra-rapid writetimes, required for example for computations by a microprocessor. Themajor difficulty with these memories is that they are volatile and thatthe relatively large size of the memory point does not enable a largestorage capacity to be obtained in a reasonable volume.

Memories of the DRAM, or dynamic RAM, type store electrical charges incapacitors, providing a large storage capacity. However, these memorieshave higher write times (several tens of nanoseconds) than those ofSRAM-type memories, and are also volatile, the information retentiontime being of around some tens of milliseconds.

Conversely, in the case of applications requiring storage of informationeven when power is off, solid-state memory devices which preserveinformation without power are also known: these devices are callednon-volatile memories. For many years, various technological solutionshave thus been developed, and have led to the availability ofnon-volatile memories which can be written and erased electrically. Thefollowing may for example be cited:

-   -   EPROMs (“Erasable Programmable Read Only Memories”), the content        of which may be written electrically, but which must be exposed        to UV radiation to erase the recorded data;    -   EEPROMs (“Electrically Erasable Programmable ROMs”), the content        of which may be written and erased electrically, but which        require larger semiconductor areas for their manufacture than        EPROM memories, and which are therefore more costly to        manufacture.

Since both solutions mentioned above have limits to their application,manufacturers have worked on finding an ideal non-volatile memory, whichcan combine the following characteristics: electrical writing anderasing, high density and low cost per bit, random access, short writeand read times, satisfactory life expectancy, but also low consumptionand low power voltage.

Many non-volatile memories also exist, called Flash memories, which donot have the shortcomings of the EPROM or EEPROM memories mentionedabove. A Flash memory is formed from multiple memory cells which may beprogrammed electrically on an individual basis, where a large number ofcells, called a block, sector or page, can be erased simultaneously andelectrically. Flash memories combine both the benefit of EPROM memoriesin terms of integration density, and the benefit of EEPROM memories interms of electrical erasure.

In addition, the durability and low electrical consumption of Flashmemories make them interesting for many applications: digital cameras,mobile telephones, printers, personal assistants, laptop computers, orportable acoustic reading and recording devices, USB flash drives, etc.In addition, Flash memories have no mechanical elements, giving themquite high impact resistance. In the “all-digital” era these productshave become very widely used, leading to an explosion of the Flashmemory market.

Most commercially available non-volatile Flash memories use chargestorage as the data encoding basis. In practice, a charge-trapping layer(generally polysilicon, or a dielectric such as SiN) is encapsulatedbetween two dielectrics in the gate stack of a MOS transistor. Thepresence or absence of a charge in this medium modifies the condition ofthe MOS transistor, and enables the state of the memory to be encoded.

More recently, other types of rewritable non-volatile memories haveappeared to reduce the voltages and programming times of Flash memories;in particular FeRAM or “Ferroelectric RAM” memories, based onpolarisation switching, or MRAM or “Magnetic RAM memories, which use thedirection of the residual magnetic field in the active material, may bementioned. However, FeRAM and MRAM memories present difficulties whichlimit their scaling-down.

To overcome these difficulties variable-resistance memories (called RRAMor “Resistive RAM” memories) are known; these are now the subject ofgreat attention. Memories of the resistive type may have at least two“off” or “on” states, corresponding to the transition from a resistivestate (“OFF” state) to a less resistive state (“ON” state).

Three types of resistive memory may be distinguished: memories using athermochemical mechanism, memories based on a change of valency, andmemories based on electrochemical metallisation.

The latter category may be based on active materials such asion-conductive materials, which can be referred to as CBRAM or“Conductive Bridging RAM” materials (or PMC materials, for ProgrammableMetallization Cell) and the operation of which is based on reversibleformation and rupture of a conductive filament in a solid electrolyte,by dissolution of a soluble electrode. These memories are extremelypromising due to their low programming voltages (of the order of oneVolt), their short programming times (<1 μs), their low consumption andtheir low integration cost. In addition, these memories may beintegrated in the metallisation levels of the logic of the circuit(“above IC”), enabling the density to be increased. From thearchitecture standpoint these memories generally require a selectiondevice, which may, for example, be a transistor or a diode.

The operation of CBRAM-type devices is based on the formation, within asolid electrolyte, of one or more metal filaments (also called“dendrites”) between two electrodes, when appropriate respectivepotentials are applied to these electrodes. Formation of the filamentenables a given electrical conduction to be obtained between the twoelectrodes. By modifying the respective potentials applied to theelectrodes the distribution of the filament may be modified, and by thisthe electrical conduction between the two electrodes may be modified. Byreversing, for example, the potential between the electrodes, the metalfilament may be made to disappear or to be reduced, so as to eliminateor substantially reduce the electrical conduction due to the presence ofthe filament. CERAM devices may thus have a two-state operation: a statecalled “ON” and a state called “OFF”, and by this means may act asmemory cells.

FIG. 1 represents a schematic diagram of an electronic device 1 of theCERAM type.

This device 1 is formed by a stack of the Metal/Ionic Conductor/Metaltype. It comprises a solid electrolyte 2, for example made of dopedchalcogenide, such as GeS, positioned between a lower electrode 3, forexample made of Pt, forming an inert cathode, and an upper electrode 4comprising a portion of ionisable metal, for example made of Ag or Cu,i.e. a portion of metal which is able easily to form metal ions (in thiscase, Ag+ or Cu2+ ions), and forming an anode. Device 1 represented inFIG. 1 typically forms a memory point, i.e. a unit memory cell, of amemory comprising a large number of these memory devices: in this casethe electrolyte is generally integrated in a “contact point”, betweenthe two electrodes, which are organised in line in mutuallyperpendicular directions.

The memory state of a CBRAM memory device results from the difference ofelectrical resistivity between two states: ON and OFF. In the OFF statethe metal ions (for example, in this case, Ag+ ions for a solubleelectrode which is in Ag) originating from the ionisable metal portionare dispersed throughout the solid electrolyte. No electrical contact isthus made between the anode and the cathode, i.e. between the ionisablemetal portion and the lower electrode. The solid electrolyte forms anelectrically insulating area of high resistivity between the anode andthe cathode.

When a positive potential V is applied to upper soluble electrode(anode) 4, an oxidation-reduction reaction takes place at thiselectrode, creating mobile ions 5 (FIG. 2).

In the case of a silver electrode 4, the following reaction takes place:Ag→Ag⁺ +e ⁻.

To accomplish this, potential V applied to soluble electrode 4 should besufficient for the redox reaction to take place.

Ions 5 then move in electrolyte 2 under the effect of the appliedelectrical field. The speed of movement depends on the mobility of theion in the electrolyte in question, which guides the choice of thesoluble electrode/electrolyte pair (examples: Ag/GeS; Cu/SiO2, etc.).The ions' speeds are of the order of one nm/ns.

When they arrive at inert electrode 3 (the cathode), ions 5 are reducedthrough the presence of electrons supplied by the electrode, leading tothe growth of a metal filament 6 according to the following reaction:Ag⁺ +e ⁻→Ag

This filament preferentially grows in the direction of soluble electrode4.

Memory 1 then changes to the ON state (FIG. 3) when filament 6 allowscontact between the two electrodes 3 and 4, making the stack conductive.This phase is called the SET of the memory.

To change to the OFF state (RESET phase of the memory), a negativevoltage V is applied to upper electrode 4, leading to the dissolution ofthe conductive filament. To explain this dissolution, thermal (heating)and oxidation-reduction mechanisms are generally invoked.

Many studies relate to these CBRAM memories in order to improve theirreliability and their performance characteristics. Among the proposedsolutions the following may in particular be cited: engineering of theelectrolyte (addition of doping agents, choice of new materials,annealing, UV processing, etc.), engineering of the soluble electrodeand of the inert electrode, or the addition of interface(s) between theelectrodes and the electrolyte.

To reduce the dimensions of CERAM memories, an architecture based onring-shaped conductive electrodes was proposed in U.S. Pat. No.8,022,547. This solution enables the dimensions of the active area to bereduced, without using critical photolithography steps.

However, the known solutions mentioned above have certain difficulties.

One of the difficulties of filament memories such as CBRAMs thus relatesto the high dispersion of certain electrical characteristics. Inparticular, high dispersions of the SET and RESET voltages are measuredin the memory matrices, but also within a given device, during cyclingof the cell (life expectancy measurement). This dispersion is importantfor the reliability of these devices, and limits their large-scaleintegration. These limitations are also found in memories of the OXRRAMtype (oxide-based resistive memories), in which the change of resistivestate is related to the formation of a filament of oxygen vacancies. Oneof the origins invoked to explain this dispersion relates to thedifficulty in controlling the size and position of the filament, whichmay vary from one cycle to the next in the memory cell.

Document US2011/0120856 describes a CBRAM memory enabling the shape ofthe filament to be controlled; to accomplish this, the electrolyte hasan asymmetrical shape, such that the contact section of the electrolytewith the soluble electrode is less than the contact section of theelectrolyte with the inert electrode. The shape of the spacerssurrounding the electrolyte, combined with the shape of the electrolyte,enable the active region, and therefore the shape of the conductivefilament, to be clearly defined.

However, this solution also has certain drawbacks.

SUMMARY

The reduction of the size of the electrolyte may thus lead to adegradation of performance when the size of the electrolyte in the areaof the soluble electrode is close or equal to that of the filament whichit is desired to create. For example, when erasing there is then littleavailable space in the electrolyte to dissolve the formed filament.

An aspect of the invention seeks to remedy the drawbacks of the state ofthe art by proposing an electronic device with improved electricalcharacteristics and cyclability.

In this context, in an embodiment of the present invention, there isprovided an electronic device comprising:

-   -   a first electrode made of an inert material;    -   a second electrode made of a soluble material;    -   a solid electrolyte made of an ion-conductive material, where        the first and second electrodes are in contact respectively with        one of the faces of the electrolyte, either side of the        electrolyte, where the second electrode is able to supply mobile        ions flowing in the electrolyte towards the first electrode, in        order to form a conductive filament between the first electrode        and the second electrode when a voltage is applied between the        first and second electrodes;        where the second electrode made of soluble material is a        confinement electrode, where the confinement electrode comprises        an end surface in contact with the electrolyte which is less        than the available surface of the electrolyte, such that        confinement of the contact area of the confinement electrode on        the solid electrolyte is obtained.

The term “conductive filament” is understood to mean at least onenanowire or dendrite (for example metallic) formed by the growth of ions(for example metal ions) within the electrolyte.

By virtue of an embodiment of the invention at least one of the twoelectrodes is subject to confinement; this confinement enables theformation of the conductive filament to be controlled. The smaller thecontact surface, the more the filament will be guided in its growth, andtherefore the more the electrical characteristics will be reproducible.In an embodiment, the confinement electrode is the electrode made ofsoluble material, such that the volume of the ion donor material isreduced. This confinement enables the size and position of the filamentto be controlled, while guiding it in its growth during the SEToperation of the CBRAM memory. Confinement of the upper electrodeenables an electrical field peak to be generated during the SEToperation, causing preferential vertical growth of the filament. Inother words, a point effect on the confinement electrode leads to alocal increase of the electrical field during the SET operation,enabling the filament to be guided during its formation, and thereforethe size and position dispersions of the filament to be reduced.

The device according to an embodiment of the invention may also have oneor more of the characteristics below, considered individually, or in alltechnically possible combinations:

-   -   the section of the confinement electrode measured parallel to        the contact plane of the electrolyte with the confinement        electrode continuously grows from the contact plane over at        least part of the height of the confinement electrode;    -   the volume of the electrolyte is chosen such that the surface of        the first end of the electrolyte in contact with the first        electrode is strictly less than the surface of the second end of        the electrolyte opposite the first end; according to this        embodiment, both the electrolyte and one of the electrodes are        confined (for example by producing two successive cavities in        dielectric materials which will receive respectively the        electrolyte and the electrode). This double confinement is still        more beneficial since it enables the size and position of the        filament to be controlled more effectively. An asymmetrical        volume is beneficially used for the electrolyte, so as to        benefit from a reduced volume on the side of the inert        electrode, and from a larger volume on the side of the soluble        electrode. It will be noted that this configuration of the        confinement electrolyte is precisely the reverse of the        configuration shown in document US2011/0120856. In other words,        in this case the shape of the electrolyte is beneficially        adjusted in order to reduce the contact surface with the inert        electrode, whilst retaining a sufficient volume of electrolyte,        notably for erasure.

By choosing such a configuration, confinement of the electrolyte in thearea of the inert electrode is used, enabling the formation of theconductive filament to be controlled (i.e. the smaller the volume ofelectrolyte, the more the filament will be guided in its growth, andtherefore the more the electrical characteristics will be reproducible),but at the same time a sufficient volume of electrolyte is also retained(notably in the soluble electrode) in order not to degrade theperformance characteristics of the device (the size of the electrolyteremains greater than that of the filament in the area of the solubleelectrode); thus, when erasing, sufficient available space remains onapproaching the soluble electrode in the electrolyte to dissolve theformed filament.

-   -   the section of the electrolyte measured parallel to the contact        plane of the electrolyte with the first electrode grows        continuously from the contact plane over at least part of the        height of the electrolyte;    -   the electrolyte has a first portion of constant section in        contact with the first electrode followed by a second portion of        constant section, where the constant section of the first        portion is less than the constant section of the second portion,        and where the constant sections are measured parallel to the        contact planes of the electrolyte with the first and second        electrodes;    -   the device according to an embodiment of the invention comprises        an insulating device or insulator surrounding the electrolyte        over at least part of its height;    -   the device according to an embodiment of the invention comprises        an insulating device or insulator surrounding the confinement        electrode over at least part of its height;    -   the solid electrolyte is manufactured from a chalcogenide        material such as a selenide or a telluride, from certain oxides        such as SiO₂, HfO₂, Ta₂O₅, TiO₂, GdOx, CuOx, WOx, or from        sulphides such as GeS_(x), Cu_(x)S or AgS.

Another aspect of the present invention is a method of manufacture ofthe device according to an embodiment the present invention comprising:

-   -   producing an aperture in an insulating material, where the        aperture has the shape of the confinement electrode;    -   filling the aperture by a material forming the electrode.

According to a first embodiment, the aperture is obtained by anisotropicetching of the insulating material.

According to a second embodiment, the aperture is obtained bytransferring a pattern of resin subject to creep in the insulatingmaterial.

Another aspect of the present invention is a method for manufacturingthe device according to an embodiment of the present inventioncomprising:

-   -   producing of a first aperture in an insulating material, where        the first aperture has the shape of the electrolyte;    -   filling the first aperture by a material forming the        electrolyte;    -   producing a second aperture in an insulating material, where the        second aperture has the shape of the confinement electrode;    -   filling the second aperture by a material forming the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and benefits of the invention will become clearfrom the description which is given of it below, by way of example andnon-restrictively, with reference to the appended figures, in which:

FIGS. 1 to 3 represent schematically an ion conduction device of theCBRAM type according to the state of the art;

FIG. 4 represents schematically an electronic device according to afirst embodiment of the invention;

FIGS. 5 to 7 represent schematically three other embodiments of thedevice according to the invention with different shapes of electrolytes;

FIGS. 8 to 15 illustrate an example embodiment of a method ofmanufacture of the device of FIG. 4;

FIG. 16 represents schematically another embodiment of the deviceaccording to the invention.

In all the figures the common elements have the same reference numbers.

DETAILED DESCRIPTION

FIGS. 1 to 3 have previously been described with reference to the stateof the art.

FIG. 4 illustrates a first embodiment of a device 100 according to theinvention.

Device 100 is formed by a stack of the Metal/Ionic Conductor/Metal type.

Device 100 comprises a solid electrolyte 112, for example made of dopedchalcogenide, such as GeS, positioned between a lower inert electrode103, for example made of Pt, forming an inert cathode, and an upperelectrode 104 comprising a portion of ionisable metal, for example madeof Ag, i.e. a portion of metal which is able easily to form metal ions(in this case, Ag+ ions), and forming an anode. This device 100 alsocomprises lateral dielectric portions 107 positioned around electrolyte112. These dielectric spacers 107 are themselves surrounded bydielectric portions 117.

Inert electrode 103 is isolated laterally (for example by dielectricspacers 115) and rests on a metal line 116. This metal line 116 may, forexample, form part of an access line (line of bits or line of words) ifresistive memory device 100 according to the invention is integrated inan architecture of the “cross-bar” type.

A first solution to control the formation of the conductive filamentwithin the electrolyte would be to reduce the size of the electrolyteuniformly. The smaller the volume of the electrolyte, the more thefilament will be guided in its growth, and therefore the more theelectrical characteristics will be reproducible. Formation of thefilament is constrained by the small size of the electrolyte, close tothe size of the filament. However, reduction of the size of theelectrolyte may lead to a degradation of performance when the size ofthe electrolyte is close or equal to that of the filament which it isdesired to create. For example, in the case of an erasure operation,there is little available space in the electrolyte to dissolve theformed filament. The shape of electrolyte 112 is beneficially chosen toreduce the contact surface 113 between electrolyte 112 and inertelectrode 103, while retaining a sufficient volume of electrolyte.

In other words, the volume of electrolyte 112 is asymmetrical, andchosen such that contact surface 113 of the electrolyte with inertelectrode 103 is strictly less than surface 114 of electrolyte 112located on the side of soluble electrode 104.

The sides of electrolyte 112 are in this case concave and rounded, witha section measured parallel to the plane of the layers of the stack (andtherefore parallel to the contact planes of electrodes 103 and 104)which grows continuously from contact surface 113 of electrolyte 112with inert electrode 103 as far as surface 114 of electrolyte 112 on theside of soluble electrode 104. The shape is given purely illustrativelyand in no way restrictively.

Soluble electrode 104 is in this case an electrode of which the end incontact with surface 114 of electrolyte 112 is small. In other words,the surface of the end of soluble electrode 104 in contact withelectrolyte 112 is less than the available surface of electrolyte 112,such that a confinement of the contact area of soluble electrode 104 onsolid electrolyte 112 is obtained. As with electrolyte 112 the sides ofsoluble electrode 104 are concave and rounded. The shape is given purelyillustratively and in no way restrictively.

Device 100 also comprises lateral dielectric portions 118 positionedaround soluble electrode 104. These dielectric spacers 118 arethemselves surrounded by dielectric portions 119.

The confinement of upper soluble electrode 104 allows the formation ofthe conductive filament to be controlled. The smaller contact surface113, the more the filament will be guided in its growth, and thereforethe more the electrical characteristics will be reproducible.

According to this embodiment, both electrolyte 112 and upper solubleelectrode 104 are confined. This double confinement enables the size andposition of the filament to be controlled, while guiding it in itsgrowth during the SET operation of the CERAM memory. Confinement of theelectrolyte enables the available space for the filament to be reduced(and therefore the filament to be guided), and confinement of the upperelectrode enables an electric field peak to be generated during the SEToperation, leading to vertical preferential growth of the filament.

By choosing such a geometry for electrolyte 112 and soluble electrode104, CERAM device 100 according to an embodiment of the invention notonly enables a localised filament to be obtained, but also a sufficientvolume of electrolyte to be kept to facilitate erasure.

The filament is thus guided in its growth through the narrowing of theend of soluble electrode 104 in contact with effective surface 114 ofelectrolyte 112 and the confinement of contact area 113 of electrolyte112 with inert electrode 103, the electrical characteristics thus beingreproducible.

The memory state of CERAM memory device 100 results from the differenceof electrical resistivity between two states: ON and OFF. In the OFFstate the metal ions (for example, in this case, Ag+ ions for a solubleelectrode which is in Ag) originating from the ionisable metal portionare dispersed throughout solid electrolyte 112. No electrical contact isthus made between soluble electrode 104 and inert electrode 103, i.e.between the portion of ionisable metal and the lower electrode. Thesolid electrolyte 112 forms an electrically insulating area of highresistivity between the anode 104 and the cathode 103.

When a positive potential V is applied to upper soluble electrode 104,an oxidation-reduction reaction takes place at this electrode, creatingmobile ions.

In the case of a silver electrode 104, the following reaction takesplace:Ag→Ag⁺ +e ⁻.

For this to occur, potential V applied to soluble electrode 104 isgreater than the redox potential of electrode 104 in question (generallyof the order of several hundred mV).

The fact that electrode 104 is confined leads to a point effect in thiselectrode, and a local increase of the electrical field created duringthis SET operation. This very localised increase of the field enablesthe Ag+ ions to be guided, and therefore enables the formation of thefilament to be guided. The Ag+ ions then move in electrolyte 112 underthe effect of the applied and localised electrical field. When theyarrive at inert electrode 103 the Ag+ ions are reduced through thepresence of electrons supplied by the electrode, leading to the growthof a metal filament according to the following reaction:Ag⁺ +e ⁻→Ag

This filament grows in the direction of soluble electrode 104.

Due to the narrowing (or confinement) of electrolyte 112 on approachinginert electrode 103, the Ag+ ions are constrained in their movement,such that formation of the filament is perfectly controlled. This is theinitial creation of the filament in the area of inert electrode 103which is controlled due to the reduced size of contact surface 113between electrolyte 112 and cathode 103.

Memory 100 then changes to the ON state when the filament allows contactbetween the two electrodes 103 and 104, making the stack conductive.This phase is called the SET of the memory.

To change from the OFF state (RESET phase of the memory), a negativevoltage V is applied to upper electrode 104, leading to the dissolutionof the conductive filament in the form of Ag+ ions. Due to device 100according to an embodiment of the invention and to the flared shape ofelectrolyte 112, a sufficient volume is retained in the upper portion ofthe electrolyte (i.e. on the side of soluble electrode 104) towardswhich the ions are directed during dissolution so as to facilitateerasure, with the ions being redeposited on soluble electrode 104.

The shape of electrolyte 112 with sides of concave, rounded shape, isgiven purely illustratively, with the understanding that otherelectrolyte shapes are perfectly conceivable; three other examples ofelectrolytes are thus illustrated in FIGS. 5 to 7.

As with electrolyte 112 of device 100, electrolytes 212 and 312,belonging respectively to CERAM devices 200 and 300 of FIGS. 5 and 6,have a shape which is chosen such that the section of the electrolytemeasured parallel to the plane of the layers of the stack (and thereforeparallel to the contact planes of electrodes 103 and 104) growscontinuously from contact surface 213 (respectively 313) of electrolyte212 (respectively 312) with inert electrode 103 as far as contactsurface 214 (respectively 314) of electrolyte 212 (respectively 312)with soluble electrode 104.

Electrolyte 212 has a general globally pyramidal shape, whereaselectrolyte 312 has sides of a rounded, convex shape. It remains thecase that the most efficient form is the one which combines a smallelectrolyte surface on the side of the inert electrode with a highvolume of electrolyte on the side of the soluble electrode.

According to another embodiment illustrated in FIG. 7, CBRAM device 400according to an embodiment of the invention comprises an electrolyte 412having a first portion 412B of constant section in contact with inertelectrode 103, and a second portion 412A of constant section in contactwith soluble electrode 104, where the sections are measured parallel tothe plane of the layers of the stack (and therefore parallel to thecontact planes of electrodes 103 and 104). The constant section of firstportion 412B is chosen to be smaller than the constant section of secondportion 412A.

The shapes of the electrolytes (cf. in particular, FIGS. 4, 5 and 6) maybe obtained by production of spacers in a via of relaxed dimensions (andwhich is therefore compatible with a standard technology). The shape ofthe obtained spacer depends on the etching conditions (known to thoseskilled in the art), which enable the slope of the electrolyte in thedielectric to be controlled.

The shapes of the electrolytes (cf. in particular, FIG. 7) may also beobtained by production of successive steps of lithography, or byself-aligned etching of materials showing a different etching speed inorder to increase the volume of electrolyte in the area of the solubleelectrode, while retaining a small contact area between the electrolyteand the inert electrode.

It will be noted that in each of the cases dielectric spacers 107(respectively 207, 307 and 407) and 118 have sides which match therespective sides of the electrolyte and of the confinement electrode. Inother words, there is no discontinuity of material. In practice, theelectrolyte and the confinement electrode are each deposited in thecavity formed by their respective spacers.

FIGS. 8 to 15 illustrate the main steps of a method of manufacture ofdevice 100 of FIG. 4.

According to step 501 represented in FIG. 8, a metal line 116 is firstproduced, for example on a Si substrate or on MOS transistors of anunrepresented lower logic level. As mentioned above, this metal line mayserve as a line of words or bits in a matrix structure of the“cross-bar” type, incorporating several memory devices according to anembodiment of the invention. This line 116 may be produced in a knownmanner by a step of etching after photolithography, and formed, forexample, from Cu or an AlCu or AlSi alloy.

According to this step 501 lower inert metal electrode 103 is alsoproduced. This electrode 103 is, for example, obtained by a method ofthe damascene type, which consists in etching a VIA, through a resinmask, in a dielectric 115. The VIA is then covered by the metal, and thesurplus metal is then removed by chemical mechanical polishing, CMP, toobtain inert electrode 103. Dielectric 115 insulating the VIA may forexample be SiO₂, deposited on the substrate, which may be between 50 and100 nm thick. Inert electrode 103 is for example made of Pt, TiN or W.

According to step 502 represented in FIG. 9 a dielectric layer (forexample a layer of Si₃N₄ which is between 50 nm and 100 nm thick) isdeposited. This dielectric layer is then etched after photolithographyso as to produce a second cavity 122 surrounded by dielectric portions117. Width c of cavity 122 is in this case less than that of metal VIA13; it will however be noted that the dimensions of this second cavityare not critical for implementation of the method.

According to step 503 represented in FIG. 10, a dielectric is depositedin cavity 122. This dielectric is etched (for example by anisotropicetching) so as to produce spacers 107 located on the sides of secondcavity 122.

The shape of spacers 107 (and in particular the sides of spacers 107)obtained depends on the etching conditions (known by those skilled inthe art), and will subsequently enable the complementary shape of theelectrolyte deposited in the dielectric to be defined.

Width t of each of spacers 107 is equal to the width (or thickness) ofthe deposited dielectric; this width t is chosen relative to width c ofsecond cavity 122 so as to leave a space 123 of width g in second cavity122 forming an aperture in lower inert metal electrode 103. Dimensionst, c and g are thus chosen so as to have a space 123 with the smallestpossible dimension, of the order of several nm: this space 123 definesthe contact surface between the electrolyte and the inert electrode. Asan example, the following triplet may be used: c=100 nm; t=45 nm; g=10nm. As mentioned above, the dimension of dielectric 117 may be relaxed:the length of second cavity 122 is not critical and may be of a largerdimension. The dielectric material forming spacers 107 is chosen suchthat the soluble metal electrode has a low coefficient of diffusion inthis dielectric material. These spacers 107 should not, indeed, act asan electrolyte. As an example, in the case of an Ag soluble electrode,spacers 107 made of SiO₂ may be chosen.

According to step 504 represented in FIG. 11, solid electrolyte 112 (forexample GeS₂ (50 nm) or SiO₂ (10 nm), etc.) is deposited.

According to step 505 represented in FIG. 12, a chemical mechanicalpolishing of the upper portion of electrolyte 112 (i.e. the portionlocated above cavity 123) is undertaken so as to obtain an electrolyte112 of lesser volume.

According to an unrepresented alternative it is also possible to obtainan aperture such as aperture 123 by not using spacers 107 and by usingother methods such as resin creep. According to this latter technique, alayer of dielectric material 117 is retained (FIG. 9) without makingaperture 118. A pattern of resin subject to creep is made in the areawhere the electrolyte is produced. This resin pattern has roughly thesame shape as the lower portion of electrolyte 112. This type of patternmay be obtained, notably, by using a resin of the “Deep UV” type whichis spread and then lithographed. A fraction of the resin is imaged (forexample by UV radiation) through a mask comprising a portion which istransparent to short-wavelength UV radiation (typically of between 150and 300 nm) and a portion which is opaque to UV radiation (absorbing UVradiation); a latent image is then created, by photochemical reaction,all the way through the photosensitive resin. The resin is then“developed”. A final annealing is applied to harden the resin patternformed in this manner. After these steps of development and annealing, atemperature above the glass transition temperature is applied to makethe resin “creep”. The shape of the resin pattern is then transferredinto the dielectric layer so as to obtain an aperture having roughly theshape of the lower portion of the electrolyte. This transfer isobtained, for example, by undertaking a plasma etching.

According to step 506 represented in FIG. 13 a dielectric layer (forexample a layer of Si₃N₄ or of SiO₂ which is between 50 nm and 100 nmthick) is deposited. This dielectric layer is then etched afterphotolithography so as to produce a cavity 120 surrounded by dielectricportions 119. Width c′ of cavity 120 is in this case less than that ofmetal VIA 13; it will however be noted that the dimensions of cavity 120are not critical for implementation of the method.

According to step 507 represented in FIG. 14, a dielectric is depositedin cavity 120. This dielectric is etched (for example by anisotropicetching) so as to produce spacers 118 located on the sides of cavity120.

The shape of spacers 118 obtained depends on the etching conditions, andwill subsequently enable the complementary shape of the solubleelectrode deposited in the dielectric to be defined.

Width t′ of each of spacers 118 is equal to the width (or thickness) ofthe deposited dielectric; this width t′ is chosen relative to width c′of cavity 120 so as to leave a space 121 of width g′ in cavity 120forming a smaller aperture on the surface of solid electrolyte 112.

Dimensions t′, c′ and g′ are thus chosen so as to have a space 120 withthe smallest possible dimension, of the order of several nm: this space120 defines the contact surface between the soluble electrode and theelectrolyte. The dielectric material forming spacers 118 is chosen suchthat the soluble metal electrode has a low coefficient of diffusion inthis dielectric material. These spacers 118 should not, indeed, act asan electrolyte. As an example, in the case of an Ag soluble electrode,spacers 118 made of Si₃N₄ may be chosen.

According to step 508 illustrated in FIG. 15, upper soluble electrode104 is deposited and etched after photolithography (for example athickness of between 10 nm and 50 nm of Cu or Ag), so as to obtaindevice 100 as represented in FIG. 4. The point effect produced in thismanner in the area of the upper electrode leads to a local increase ofthe electric field during the SET operation, which enables the filamentto be guided and its position to be controlled, and therefore thefilament's size and position dispersions to be reduced.

FIG. 16 illustrates another embodiment of a device 600 according to anembodiment of the invention. The difference between device 100 of FIG. 4and device 600 of FIG. 16 lies in the fact that the electrolyte ofdevice 600 is not confined, and only the soluble electrode is confined.

As with device 100, device 600 comprises a solid electrolyte 612positioned between an inert lower electrode 603 and an upper electrode604.

Device 600 is formed by a stack of the Metal/Ionic Conductor/Metal type.

Device 600 comprises a solid electrolyte 612, for example made of dopedchalcogenide, such as GeS, positioned between a lower inert electrode603, for example made of Pt, forming an inert cathode, and an upperelectrode 604 comprising a portion of ionisable metal, for example madeof Ag, i.e. a portion of metal which is able easily to form metal ions(in this case, Ag+ ions), and forming an anode. This device 600 alsocomprises lateral dielectric portions 617 positioned around electrolyte612.

Inert electrode 603 is isolated laterally (for example by dielectricspacers 615) and lies on a metal line 616.

In this case, electrolyte 612 has a standard shape without confinement(i.e. a roughly parallelepipedic shape).

In other words, the volume of electrolyte 612 is symmetrical in thiscase, and chosen such that contact surface 613 of the electrolyte withinert electrode 603 is roughly equal to available surface 614 ofelectrolyte 612 located on the side of soluble electrode 604.

In accordance with an embodiment of the invention, soluble electrode 604is an electrode of which the end in contact with surface 614 ofelectrolyte 612 is small. In other words, the surface of the end ofsoluble electrode 604 in contact with electrolyte 612 is less than thetotal available surface of electrolyte 612, such that a confinement ofthe contact area of soluble electrode 604 on solid electrolyte 612 isobtained. The sides of soluble electrode 604 are concave and rounded.The shape is given purely illustratively and in no way restrictively.

Device 600 also comprises lateral dielectric portions 618 positionedaround soluble electrode 604. These dielectric spacers 618 arethemselves surrounded by dielectric portions 619.

Device 600 may be obtained according to a method comparable to the onedescribed with reference to FIGS. 8 to 15, where only the steps relativeto the shaping of a confined electrolyte are eliminated.

It will be appreciated that the device and the method according to theinvention are not limited to the embodiments which have been describedby way of examples and in no way restrictively with reference to FIGS. 1to 16.

Although the invention has been described illustratively using onefilament it is thus understood that the device according to theinvention may comprise multiple filaments distributed in theelectrolyte.

It will also be noted that the device contains a soluble electrode andan inert electrode, where each of these electrodes may equally bepositioned as the upper or lower electrode, where the inert electrodeis, in an embodiment, preferably positioned on the side of the narrowestarea of the electrolyte, and where the soluble electrode is positionedon the side of the large volume of the solid electrolyte.

The invention claimed is:
 1. An electronic device comprising: a firstelectrode made of an inert material; a second electrode made of asoluble material for supplying mobile ions; a solid electrolyte made ofan ion-conductive material, wherein the first and second electrodes arein contact respectively with one of the faces of the electrolyte, eitherside of said electrolyte, wherein, when a voltage is applied between thefirst and second electrodes, the second electrode supplies the mobileions flowing in the electrolyte towards the first electrode, in order toform a conductive filament between the first electrode and the secondelectrode; wherein the second electrode made of soluble material is aconfinement electrode, wherein said confinement electrode comprises anend surface in contact with the electrolyte which is less than theavailable surface of the electrolyte, such that confinement of a contactarea of said confinement electrode on the solid electrolyte is obtained,and wherein the volume of the electrolyte is chosen such that a surfaceof a first end of the electrolyte in contact with the first electrode isstrictly less than a surface of a second end of the electrolyte oppositethe first end.
 2. A device according to claim 1, wherein a section ofthe confinement electrode measured parallel to a contact plane of theelectrolyte with the confinement electrode continuously grows from thecontact plane over at least part of the height of said confinementelectrode.
 3. A device according to claim 2, wherein the section of theelectrolyte measured parallel to the contact plane of said electrolytewith the first electrode grows continuously from the contact plane overat least part of the height of the electrolyte.
 4. A device according toclaim 2, wherein the electrolyte has a first portion of constant sectionin contact with the first electrode followed by a second portion ofconstant section, wherein the constant section of the first portion isless than the constant section of the second portion, and wherein theconstant sections are measured parallel to the contact planes of theelectrolyte with the first and second electrodes.
 5. A device accordingto claim 1, comprising an insulator surrounding the electrolyte over atleast part of its height.
 6. A device according to claim 1, comprisingan insulator surrounding the confinement electrode over at least part ofits height.
 7. A method of manufacture of the device according to claim6, comprising: producing an aperture in an insulating material, whereinsaid aperture has the shape of said confinement electrode; filling saidaperture by a material forming the confinement electrode.
 8. A methodaccording to claim 7, wherein said aperture is obtained by anisotropicetching of the insulating material.
 9. An electronic device comprising:a first electrode made of an inert material; a second electrode made ofa soluble material for supplying mobile ions; a solid electrolyte madeof an ion-conductive material and extending from a first end to a secondend, opposite the first end, wherein the first end of the solidelectrolyte is in contact with the first electrode and the second end ofthe solid electrolyte is in contact with the second electrode; wherein across-section of the solid electrolyte varies from the first end to thesecond end so that a surface area of the first end in contact with thefirst electrode is lower than a surface area of the second end incontact with the second electrode and a surface area of the secondelectrode that is in contact with the solid electrolyte is lower thanthe surface area of the second end of the electrolyte, and wherein theelectronic device has a conductive state in which, when a voltage isapplied between the first and second electrodes, the second electrodesupplies the mobile ions flowing in the electrolyte towards the firstelectrode to form a conductive filament between the first electrode andthe second electrode.
 10. A device according to claim 9, wherein thesolid electrolyte has a first portion of constant section in contactwith the first electrode followed by a second portion of constantsection, wherein the constant section of the first portion is less thanthe constant section of the second portion, and wherein the constantsections are measured parallel to the contact planes of the electrolytewith the first and second electrodes.
 11. A device according to claim 9,comprising an insulator surrounding the solid electrolyte over at leastpart of its height.
 12. A device according to claim 9, comprising aninsulator surrounding the second electrode over at least part of itsheight.
 13. A device according to claim 9, wherein the solid electrolyteis made of a chalcogenide material, or at least one oxide selected fromthe group consisting of SiO₂, HfO₂, Ta₂O₅, TiO₂, GdOx, CuOx, and WOx, orat least one sulphide selected from the group consisting of GeS_(x),Cu_(x)S and AgS.